Project II- Project Summary The mirror neuron system (MNS) has been hypothesized to broadly influence social cognitive development, but research has just begun to evaluate the nature and significance of these effects early in life. The proposed work in Project II seeks to elucidate the learning mechanisms that support early motor learning and social perception as well as the broader role that the MNS may play in supporting social-cognitive development. These issues are addressed in three aims, each of which involves recruiting behavioral data as well as electroencephalogram (EEG) measures of MNS function supported by Core B: Aim 1: Investigate the neural correlates and behavioral consequences of action learning in infants. Our prior results suggest that the MNS is plastic during early development, changing as a function of long-term motor experience, and that this plasticity may involve both long-term and short-term learning effects. Aim 1 investigates the neural and behavioral correlates of short- and long-term action training in order to shed light on how the processes involved in the initial stages of action learning relate to those that emerge with longer-term expertise as well as the behavioral functional significance of MNS activity at each timescale. Aim 2. Investigate the role of the MNS in supporting infants? social interactive competence. A foundational set of social abilities emerges in the second year, including imitative learning, helping, and communicative skill, and these skills require rapid responses to others? goal-directed actions. Research with adults has shown that the MNS supports rapid responses to others? actions, raising the possibility that this may also occur during early development. To investigate this possibility Aim 2 will investigate the effects of action priming on infants? imitation, helping, and communicative behavior, as well as the neural correlates of these social behaviors. Aim 3. Investigate longitudinal relations between early MNS activity and later social abilities. As yet, the longer-term developmental implications of MNS activity have not been studied in human infants. In Aim 3, we will follow infants longitudinally from 12 to 30 months, assessing individual variation in early MNS activity as a predictor of later-emerging social skills. Across these three aims, we will consider the role that the social context may play in modulating the MNS and the functions it supports. Most research to date has focused on the effects of motor experience. However, emerging evidence suggests that social experience may also be an important modulator of the MNS. Social interventions, and measures of social interaction with caretakers, will be used to evaluate the potential effects of social experience on the MNS and the social-cognitive functions it may support.